1. Field of the Invention
The present invention relates to a process for the production of dimethyl 2,6-naphthalenedicarboxylate and a process for the production of naphthalenedicarboxylic acid. More specifically, it relates to a process for the production of a process for the production of high-purity dimethyl 2,6-naphthalenedicarboxylate (to be sometimes referred to as "2,6-NDCM" hereinafter), in which 2,6-naphthalenedicarboxylic acid (to be sometimes referred to as "2,6-NDCA" hereinafter) obtained by liquid phase oxidation is esterified with methanol and then resultant ester is purified (to be referred to as "present specification 1" hereinafter), and a process for the production of naphthalenedicarboxylic acid (to be sometimes referred to as "NDCA" hereinafter) by oxidizing dialkylnaphthalene and/or its oxide derivative, in which process naphthalenedicarboxylic acid having an improved crystal grain size is produced (to be referred to as "present invention 2").
2. Prior Art of the Invention
NDCA and NDCM, 2,6-NDCA and 2,6-NDCM in particular, are useful materials as materials for a high-function polyester.
Related art to the present invention 1 will be explained below.
2,6-NDCA is obtained by a method in which 2,6-dialkylnaphthalene is oxidized in a solvent containing a lower aliphatic carboxylic acid in the presence of an oxidation catalyst containing cobalt, manganese and bromine (JP-B-34-2666 and JP-B-56-3337).
2,6-NDCA obtained by the above method contains organic impurities such as trimellitic acid, 6-formyl-2-naphthoic acid, etc., and heavy metals such as cobalt, manganese, etc., from the catalyst. 2,6-NDCA can be suitably used as a material for a high-function polymer only after the above impurities, etc., are removed. Since, however, 2,6-NDCA has a low solubility in a solvent and is decomposed at its melting point, it is difficult to purify 2,6-NDCA while it remains intact.
There have been therefore proposed many methods in which 2,6-NDCA is esterified with methanol to form 2,6-NDCM and then 2,6-NDCM is distilled and/or recrystallized. For obtaining particularly high-quality 2,6-NDCM, it is effective to employ a method in which the purification is carried out by a combination of distillation and recrystallization (JP-A-50-116461, U.S. Pat. No. 5,262,560, JP-B-57-35697 and JP-B-46-9697).
Further, the above heavy metals such as cobalt, manganese, etc., contained in 2,6-NDCA are expensive, and it is industrially preferred to recover and recycle them.
The method of recovering oxidation catalyst metals contained in 2,6-NDCA is largely classified to a method in which 2,6-NDCA crystals are washed with a solvent such as a mineral acid aqueous solution or an alcohol containing an acid component to elute a metal content and the metal content is recovered (JP-A-62-212345 and JP-A-5-253496) and a method in which 2,6-NDCA is esterified and the heavy metals are recovered from the reaction mixture (JP-A-3-223233 and JP-A-4-364152).
When the former method in which 2,6-NDCA crystals are washed with a solvent to elute catalyst metals is used as a method for recovering the oxidation catalyst contained in 2,6-NDCA, there is required an apparatus for separating a wash liquid and the 2,6-NDCA crystals. There is another method for recovering a metal content from a wash liquid, in which a carbonic acid compound is added to form an insoluble carbonate. However, there is required a solid-liquid separation apparatus for separating a precipitate of the above insoluble carbonate, and the apparatus is complicated. Further, the ratio of removal of the metal content by the washing is not so high, so that the washed 2,6-NDCA still contains a large amount of metals, and it is further required to remove the metal content by some other purification method.
On the other hand, in the method of JP-A-3-223233 which uses means of recovering the catalyst metals from a crude ester obtained by esterification of 2,6-NDCA, the oxidation catalyst metals are dissolved in a solvent by using, as a catalyst, a mineral acid such as sulfuric acid for the esterification. Then, a compound which generates carbonate ion is added to mother liquor from which the esterification product is separated, and the catalyst metals are recovered in the form of an insoluble carbonate. The above method has problems that reactor materials are limited due to the corrosiveness of the mineral acid such as sulfuric acid and that it is required to treat waste acid by neutralization. The above method is therefore not suitable for use in a large-scale industrial plant.
In the method disclosed in JP-A-4-364152, insoluble oxidation catalyst metals are separated from the esterification product in a molten state or a solution thereof in methanol by filtration or centrifugalization.
For maintaining the esterification product in a molten state, however, it is required to heat the esterification product to a temperature equivalent to, or higher than, the melting point (about 190.degree. C.) of 2,6-NDCA, and it is technically very difficult to separate and recover the insoluble catalyst metals from the molten liquid by filtration and centrifugalization on an industrial scale.
In the method of separating the insoluble oxidation catalyst metals from the methanol solution, the solubility of 2,6-NDCM is very low at the boiling point of methanol as shown in Examples to be described later, and it is therefore required to heat the methanol solution to a temperature higher than the boiling point of methanol under elevated pressure in order to dissolve the esterification product containing 2,6-NDCM as a main component in an industrially practical amount of a solvent. Further, it is also required to separate the catalyst metals under the above elevated pressure, and the cost for facilities required therefor increases.
Further, the present inventor's studies have revealed the following. A crystal of 2,6-NDCM obtained by recrystallization from the methanol solution has the form of scales and has a small bulk density, and it brings a large amount of mother liquor into a cake when the solid-liquid separation is carried out by a method of filtration or centrifugalization, so that no sufficient effect of removing impurities can be obtained.
Related art of the present invention 2 will be explained below.
There have been proposed various methods for producing NDCA by oxidizing dialkylnaphthalene and/or its oxide derivative with molecular oxygen. However, the crystal of crude NDCA obtained in any one of these methods has a small diameter. Further, since NDCA has a low solubility in a reaction solvent, it is difficult to increase the particle size of NDCA much even if multi-staged crystallization is carried out in the same manner as in the process for the production of terephthalic acid. There is therefore a problem that solid-liquid separation, particularly, solid-liquid separation with an industrially advantageous centrifugal separator, is very difficult in the step of separating reaction mother liquor or the step of washing subsequent thereto.
For increasing the size of the above NDCA and improving its separability, JP-A-50-121225 discloses a method in which a slurry obtained after the reaction is maintained at a temperature between 20.degree. C. and 100.degree. C. for at least 4 hours for crystal aggregation.
Further, JP-A-6-65143 discloses that the particle diameter of NDCA is increased by carrying out the oxidation in a specific temperature range (180 to 220.degree. C.).
JP-A-6-293697 discloses that fine crystals of NDCA contained reaction mother liquor and a filtrate from the washing is again fed to an oxidation reactor so that the fine crystals are grown so as to have a particle size suitable for solid-liquid separation.
Generally, since NDCA formed by the oxidation has a very small particle size, it is difficult to separate a crystal of NDCA and a solvent. Further, cobalt and manganese used as a reaction catalyst are inevitably brought into the step of producing NDCM, and their recovery is required.
According to the studies the present inventors have made, in the methods of increasing the particle size of NDCA crystals, described in JP-A-50-121255 and JP-A-6-65143, the obtained NDCA crystal are crystals having the form of a plate, a strap or an aggregate of plates or straps, and the crystals are easily crushable and are easily crushed into fine pieces when the slurry is transported with a pump, etc., so that it is difficult to fully separate the crystals.
Further, it has been found that when reaction mother liquor containing fine particles is repeatedly continuously recycled to an oxidation reactor according to the method described in JP-A-6-293697, the amount ratio of fine crystals in a slurry fed to the solid-liquid separation step gradually increases, which results in no performance of solid-liquid separation.